Militants fired several mortars or rockets at Baghdad's fortified Green Zone government district shortly after US Vice President Joe Biden flew in, underlining the fragility of Iraq's security gains.

Iraqi police said one mortar round killed two Iraqi civilians and wounded five when it blasted an apartment block in the zone. Another two rounds landed near the sprawling US embassy compound, but did not hit it.

Police had no further reports of casualties.

Biden had met with the US ambassador, Chris Hill, and the senior US military commander, General Ray Odierno, just before the mortar strikes. He was safe in an undisclosed location, aides said, his whereabouts kept secret for security reasons.

A briefing for journalists by Hill and Odierno was  interrupted by the sound of explosions. A loudspeaker at the embassy broadcast a warning to duck and take cover.

The US military said it knew of only one blast from incoming fire, which hit near the Green Zone but not inside it.

It was Biden's second trip to Iraq in three months, and the visit signalled that the Obama administration is anxious to resolve long-standing disputes between Kurdish, Shi'ite and Sunni Arab communities over land and oil that US officials fear could yet rip apart the country.

After his meeting with Hill and Odierno, Biden said a national election in January was the key to resolving those differences, which have in recent weeks been on display through public quarrelling over who was to blame for bomb attacks.

"I think a successful election is the necessary condition for those outstanding political questions to be resolved ...most of the parties ... feel the same way," Biden said.

Violence has dropped sharply in Iraq since the height of a wave of sectarian killings in 2006, due in part to a so-called "surge" of tens of thousands of US troops, but the security gains have not been matched by much political progress.

"I don't think necessarily the result of a failed political process is civil war," Hill said. "The threat is the political process will not give the country sufficient cohesion to work on its economic issues and otherwise to become a strong and stable factor in the region."         

US troops to leave

Iraq's security gains remain fragile, as evidenced by Tuesday's rocket and mortar attacks and two giant truck bombs on August 19 that killed 95 people at the foreign and finance ministries and shattered public confidence in Iraq's security.

Iraq's Shi'ite Prime Minister Nuri al-Maliki has accused  supporters of Saddam Hussein's outlawed Baath party of plotting the attacks from neighbouring Syria, triggering a diplomatic clash with Damascus that Iraq's president, an ethnic Kurd, and two vice presidents have sharply criticised.

"We do know there are some expat settlements that are inside Syria that are funding operations inside Iraq," Odierno said.

"We have to continue to press Syria to take actions against some of these elements that continue to attempt to create instability inside Iraq."

Odierno also said he continued to see the security situation in Iraq as stable, shortly before the first loud explosion interrupted his remarks.

Since 2006, Washington has pressed Iraq's Kurdish, and Shi'ite and Sunni Arab leaders, with little success, to put aside differences and compromise on issues such as a new oil law to manage the world's third-largest oil reserves.

But with U.S combat operations due to end in Iraq by August 2010, the US is running out of time and influence among Iraq's leaders to achieve its goal -- to leave behind a relatively stable Iraq. A security pact with Iraq requires all US troops to leave by 2012.

There is no longer any appetite in Washington for the Iraq war. The Obama administration is preoccupied with the deteriorating security situa